1. Field of the Invention
The invention relates to a method and an apparatus for the measuring of structures in a fingerprint or the like, comprising the measuring of chosen characteristics of the surface of the fingerprint, e.g. capacitance or resistivity, using a sensor array comprising a plurality of sensors, positioned in contact with, or close to, the surface.
2. Description of the Related Art
Identification by the use of fingerprints has lately come to the fore as a result of the increasing needs for security relating to, for example, credit cards or computer systems as well as the greatly increased availability of pattern recognition algorithms. Some systems for recognition of fingerprints have already been made available on the market. The techniques used to register the fingerprint vary.
Some of the previously known solutions are based upon optical technology using light with one or more wavelengths. These are sensitive to dirt and contamination, both in the fingerprint and on the sensor surface, and thus cleaning is necessary for both.
Another alternative is pressure measurement, such as is described in U.S. Pat. No. 5.559.504, U.S. Pat. No. 5.503.029 and U.S. Pat. No. 4.394.773. This, however, has the disadvantage that the sensor surface becomes sensitive to mechanical wear and damage, as the sensor has to have an at least partially compliant surface.
Temperature sensors have also been suggested, for example in U.S. Pat. No. 4,429,413 and international patent application PCT/NO96/00082.
Fingerprint sensors may be exposed to long term use under varying and sometimes demanding conditions. The sensor therefore needs to have a robust surface and to be as insensitive to pollution in the fingerprint and on the sensor as possible. It must be capable of reading most fingerprints without being disturbed by latent prints from earlier use, and also be capable of imaging so-called xe2x80x9cdry fingersxe2x80x9d that represent a problem for optical sensors. In some cases, e.g. in credit cards or computer keyboards, it would also be advantageous if the sensor could be made compact.
In the view of costs there is also a demand for simplicity and minimizing of the number of parts.
It is an object of the present invention to provide a sensor being easy to produce, making them cheap in production, and also relatively small.
In addition to the solutions mentioned above the measuring of capacitance has been tried as a method to measure finger prints. Examples are shown in U.S. Pat. No. 4.353.056 and U.S. Pat. No. 5.325.442. While the ridges of the fingerprint touch the sensor surface the valleys have a small distance to the sensor surface, resulting in a difference in capacitance and/or conduction measured at the different sensors. Humidity may affect the measurements, but if it is evenly distributed throughout the fingerprint an analysis of the contrast between the measurements can provide a picture of it.
All the solutions mentioned above are based upon two-dimensional sensor arrays with dimensions comparable to the size of the fingerprint. These are expensive and difficult to produce, since they comprise a large number of sensors simultaneously measuring the surface.
Also, the known sensors disclose only flat sensor surfaces. As finger prints are curved surfaces the sensors are only capable of measuring a small part of the finger print, and are thus sensitive to the angle with which the fingerprint is held against the surface.
The present invention provides a method and an apparatus for the measuring of structures in a fingerprint or the like, for example using one of the techniques described above, and characterized as stated in the disclosed claims.
As the surface of the sensor array is small, and contains few sensors compared to the known solutions, it is inexpensive and relatively simple to make. As the fingerprint to be measured is moved past the sensor array it is selfcleaning and there is no latent fingerprint remaining from the previous user, giving another advantage in relation to the known finger print sensors.
The curved or essentially U-shaped line of sensors may be made in one piece or by combining two or three linear sensor arrays. Examples of such linear arrays are described in EP 735.502.
Since the details in the fingerprints are small, it is also difficult to make the sensors of the detector small enough. In a preferred embodiment the apparatus and method according to the invention comprises two or more parallel lines of measuring points, each line of measuring points being shifted in the longitudinal direction with a distance less than the distance between the measuring points, the sensor array comprising two or more parallel lines of equally spaced sensors, preferably shifted in the longitudinal direction of the sensor array. This provides a possibility to measure structures in the fingerprint smaller than the spacing of the sensors. This is not possible with any of the previously known detector systems.
Thus, it is to be understood that the term xe2x80x9cessentially one-dimensional arrayxe2x80x9d here refers to an array having a length being much larger than its width, and may comprise more than one line of sensors.